The principal objective of the study is to determine prospectively the relationship between breast cancer risk and measures of endocrine function, viz., serum measurements of estrogens (especially "free" estradiol), progesterone, prolactin and saliva measurements of progesterone. This study will have the advantage over most previous studies in that serum samples will be obtained before clinical disease is present and that multiple samples will be obtained. For premenopausal women serial saliva progesterones will be obtained over the full luteal phases of multiple menstrual cycles, to document possible luteal abnormalities. About 17,500 women between the ages of 35 and 64 from a breast cancer screening clinic in New York City will be enrolled in the study. Breast cancers will be ascertained as completely as possible (including "interval" cases as well as those detected at screening), and pathology reports will be obtained to verify the diagnosis. Brief questionnaires on breast cancer risk factors and on hypothesized dietary promoters or inhibitors of breast cancer will also be given to all subjects. More extensive questionnaires on these factors will be administered on a case-control basis. Serum and saliva samples will be frozen and stored so that assays can be conducted in a case-control format. Among premenopausal women the blood samples will be drawn between the 19th and 24th days of the menstrual cycle, to provide luteal-phase progesterone samples. The blood- and saliva-progesterone assays, plus a record on onset of next menses, will permit interpretation of progesterone status. The multiple menstrual cycles assessed will provide a more accurate assessment of endocrine function than is available in most studies. It is anticipated that this study will provide the most definitive data available on the relationships among endocrine function, epidemiologic risk variables and breast cancer. It could also provide new leads for targeted breast-cancer screening or for preventive interventions. The availability of frozen serum specimens from a substantial number of breast cancer cases and controls will also make it possible to assess additional hypothese when new concepts and/or assays are orginated; as such it represents a valuable resource for the future.